This would be a very short-sighted thinking. ROI of making your toilets public feels like it would be much bigger than of any money spent on advertising. Here you have potential customers actually present in your store.

Also, for example I remember exactly where every McDonald's and KFC in my town's centre is only because I sometimes need them as an emergency toilet and/or free Wi-Fi provider. Guess what places come to my mind first when I'm hungry and in the area?